Reduction of energy consumption in display devices when the display device is not used, is known, for example, in monitors for personal computers and/or laptops. In such a system, the activity of a user is measured, for example, by detecting key-strokes of the user or mouse-movements of the user. In case the user has not stroke a key for some predetermined time, or has not moved the mouse of the computer and/or laptop for some predetermined time, the display device enters a so called energy reduction mode which may also be called a “sleep mode” in which the display device switches off, or in which the backlight of the display device switches off. However, often the display device switches to the energy reduction mode while the user is, for example, reading a lengthy text on the display device, not striking a key or moving the mouse.
In more advanced energy consumption schemes, the actual presence of a user in front of the display device is sensed. This has as an advantage that the display device does not enter the energy reduction mode as long as the user is in front of the display device. Furthermore, when the user moves away from the display device, the predetermined time after which the energy reduction mode is activated may be reduced, thus further reducing the (unnecessary) power consumption of the display device. Such systems are, for example, known from the US patent application US 2003/0051179 in which a display having a sensor for detecting the absence of a user is disclosed. Alternatively, EP 0 949 557 discloses a similar system in a portable computer. Both documents disclose infrared or ultrasound sensors to detect the presence or absence of a user in front of the display device. The known sensors used in the display device transmit a signal using a transmitter and sense a reflected signal from the user. Alternatively, the US patent application US 2003/0122777 discloses that the sensor may include a camera, SONAR or RADAR systems to detect a distance of the user in front of the display device.
A disadvantage of active infrared and ultrasound sensors are their dependency with respect to the material of the object. Clothing may absorb the transmitted energy too much thereby reducing the detection range or give false distance readings. Also the field of view is often relatively narrow resulting in a sensitive area only in the direct centre line in front of the display device. This may be solved by using more sensors directing in different directions—however this results in a relatively expensive solution. Passive infrared sensors will not react on stationary (warm) objects, so when a person doesn't move enough in front of the monitor, the energy reduction mode will be activated which again may be an unwanted situation.
An alternative sensor for sensing the presence of a person in front of the display device is via a capacitive sensor. The capacitive sensor is, for example, disclosed in U.S. Pat. No. 6,486,681 in which a measuring circuit for a capacitive sensor is disclosed for distance measurement and/or space monitoring. The measuring circuit comprises a phase-dependent rectifier arrangement connected to an analog-to-digital converter.
A drawback of the known capacitive sensor is that the registration of a person by the known capacitive sensor is not reliable enough.